[#371] Add custom Source IP header configuration

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

api/router_mock_test.go

@@ -64,6 +64,10 @@ type middlewareSettingsMock struct {
 	aclEnabled    bool
 }
 
+func (r *middlewareSettingsMock) SourceIPHeader() string {
+	return ""
+}
+
 func (r *middlewareSettingsMock) NamespaceHeader() string {
 	return FrostfsNamespaceHeader
 }